This invention relates to a process for the production of citric acid by fermentation. In particular it relates to a process for the production of citric acid in the free acid form, thus permitting its direct crystallization from the fermentation broth.
Because of its ease of assimilation, palatability and low toxicity, citric acid is one of the most commonly used acids in the food and pharmaceutical industry. It is widely used as an acidulant in beverages and also as an anti-oxidant for inhibiting rancidity in fat and oils. Both the free acid and its salts are employed as buffers in the preparation of jams, jellies, and gelatin preprations, and are also used as stabilizers in various food products.
Most of the world's supply of citric acid is produced by fermentation processes, generally using selected strains of Aspergillus niger with carbohydrates like molasses and dextrose as the main source of assimilable carbon. While these fermentation processes with Asperigillus niger are attractive, many difficulties are experienced. For example, over a period of time the citric acid producing capability of the Aspergillus niger culture tends to degenerate. Of more inportance is the fact that the citric acid is recovered in the form of a salt, which must then be acidified and converted to the free acid. Furthermore, a relatively long period of time, generally more than 7 days, is required for production of large quantities of citric acid by such fermentations. This long fermentation time and the extra step of converting the citrate salt to the free acid are major cost factors in producing citric acid. Thus, it is obvious that the development of a rapid fermentation process for the direct production and recovery of citric acid is of considerable commercial importance.
Belgian Pat. No. 716,247 teaches a process for the production of citric acid, using as inoculants various yeasts of the genus Candida which are capable of accumulating citric acid and of assimilating hydrocarbons, and fermenting at a pH of about 4 to about 7.5 until a substantial amount of citric acid is accumulated in the broth. Here again, however, the citric acid is produced as calcium citrate and must be acidified, usually with sulfuric acid, to convert to the more desirable free citric acid.
There are other reports in the literature describing the propagation of members of the genus Candida in media containing hydrocarbons, e.g., I. Tanabe, J. Okada, and H. Ono, Agr. Biol. Chem., 30, 1175 (1966), E. J. Nyns, J. P. Auquiere, N. Chiang, and A. L. Wiaux, Nature, 215, 177 (1967); M. J. Klug and A. J. Markovetz, Appl. Microbiol., 15, 690 (1967), J. Bacteriol., 93, 1847 (1967). However, apparently no one has heretofore succeeded in producing free citric acid in such hydrocarbon systems. It is a characteristic property of most citric acid-accumulating strains of Candida that the polyhydric alcohols erythritol, arabitol and mannitol are co-produced in the fermentation broth with the citrates. An additional distinguishing feature of the present invention is that the superior citric acid-accumulating properties of the new strain of Candida lipolytica are accompanied by co-production of only low amounts of erythritol and at most of only traces of arabitol and mannitol. Thus, the increased concentration of citric acid, present in the free form at the low terminal pH of the fermentation broth, and the small amount of co-produced impurities, in combination, allow for the isolation of citric acid directly from the filtered fermentation broth.